It is common to switch reactors into and out of transmission circuits on electric power systems. Reactors are principally used as voltage regulators for long transmission lines, such as high voltage and extra high voltage transmission lines, during low load periods. Additional uses are for load flow control, fault current limiting, and filtering. When used as a voltage regulator, a reactor is typically switched into and out of an electric power transmission circuit on a daily basis, typically every night when the loads are low, which is a significantly higher switching frequency than experienced by fault clearing switches, such as circuit breakers and sectionalizing switches. A device known as a circuit interrupter (also called a switcher) is used to switch reactors, capacitors and various types of loads into and out of their associated electric power circuits. Many types of circuit interrupters have been developed with unique design characteristics for specific electrical applications, voltage and current levels. Example circuit interrupters are described in commonly-owned U.S. Pat. Nos. 7,115,828; 7,078,643; 6,583,978; 6,483,679; 6,316,742 and 6,236,010 and U.S. application Ser. No. 11/944,111, which are incorporated by reference.
A challenging voltage regulation application, and an important application for the present invention, is reactor switching at high voltage and extra high voltage transmission levels. In these applications, current reignitions across the arc gap in the circuit interrupter cause very steep voltage increases in the reactor between turns. This electrical stress caused by reignition during reactor switching is similar to that caused by lightning only that it occurs much more frequently. That is, reactor switching typically occurs on a daily basis, whereas lightning typically occurs much less frequently, such as a yearly basis in general. The daily operation and frequent reignitions during reactor switching cause cumulative damage to reactors that reduce the life of these expensive devices. Frequent reignitions can also cause damage to the interrupter by puncturing the nozzle materials, usually Teflon®, which in turn increases the likelihood of further reignitions. Also, the strong pressure developed in interrupters can force the current to zero prematurely, which further increases the voltage on the reactor thus requiring the interrupter to withstand even higher voltages. It is therefore important to switch the reactors in a manner that minimizes damages caused to the reactors, the interrupters, and other electric system components, from reignitions occurring during the switching process.
Switching of capacitor banks is also a common occurrence in electric power systems. The inductive reactance of motors in home and industrial use cause less than unity power factors, which if uncorrected can increase system losses and cause voltage levels delivered to end-use customers to drop to unacceptable levels. Capacitor banks are usually switched into the electric power circuits during high levels of inductive loading, typically during the daylight and early evening hours when most people are awake and using electric power, to correct the power factor, reduce delivery losses, and boost the voltage to the end-use customers. Once the high level inductive loading subside, typically at night, the capacitor banks are switched out of the electric power circuit. Daily cyclical use of capacitors is therefore a common practice to balance the capacitive reactance with inductive loads, and thus minimizing the stated problem, as electric loads increase and decrease on a daily basis.
Because inductive residential loads typically increase and decrease on a daily cycle, capacitor switching in response to residential loads typically occurs on a daily basis. Capacitor switching can also occur multiple times daily, for example when residential loads are combined with industrial or municipal loads that occur at night or multiple times per day. Coal mining equipment, aluminum smelters, manufacturing assembly lines, municipal water pumps, and electric transportation loads, to name but a few examples, can place large, cyclical or intermittent inductive loads on an electric power system. As a result, capacitor switches often experience several hundred to several thousand operations per year. Circuit breakers that are designed to operate in response to overload and other emergency conditions, by comparison, typically operate much less frequently, on the order of only a few isolated operations up to a couple of dozen times per year.
Switching a capacitor bank out of an electric power circuit can cause a restrike to occur across the arc gap inside the circuit interrupter, which can cause system disturbances and damage to the capacitor bank, the circuit interrupter, and other electric system components. Restrikes during capacitor switching are similar to reignitions during reactor switching in that they both involve high voltage causing a flash-over across the arc gap between the contacts of the interrupter after the arc has been initially extinguished at a current zero-crossing. Flash-over can also occur when switching loads, lines and other types of electrical components that have significant inductive or capacitive components. Because the voltage in an electric power system is alternating, the current extinguished periodically at each current zero-crossing and the voltage periodically builds to its peak magnitude each half cycle, the voltage tends to cause a flash-over as the voltage approaches its maximum magnitude each half cycle. Each time the current flashes over as the arc gap widens on the opening stroke, the flash-over occurs as a higher voltage. A flash-over occurring across a relatively high voltage across a relatively wide contactor gap during an opening stroke of the interrupter can damage the interrupter, damage the switched device, and cause an undesirable disturbance on the electric power circuit.
A need therefore exists for circuit interrupters for reactor, capacitor, load and line switching at distribution and transmission voltages up to high voltage and extra high voltage levels that that minimizes damages caused to electric system components from flash-over across the interrupter arc gap during the switching process. In particular, because reactors used for voltage regulation are operated relatively frequently and at high transmission voltages, it is important to switch the reactors in a manner that minimizes damages caused to the reactors and the circuit interrupters from flash-over during the switching process. There is, therefore, a continuing need for a circuit interrupter suitable for switching reactors on high voltage and extra high voltage transmission lines to extend the life of the reactors and the reactor switching circuit interrupters by minimizing the frequency of high voltage flash-over that can causes damage and life reduction.